package org.base23.commons.utils;

public class SnowflakeUtil {

  // 起始时间戳
  private final static long START_STMP = 1480166465631L;

  // 每部分的位数
  private final static long SEQUENCE_BIT = 12; // 序列号占用位数
  private final static long MACHINE_BIT = 5; // 机器id占用位数
  private final static long DATACENTER_BIT = 5; // 机房id占用位数

  // 每部分最大值
  private final static long MAX_DATACENTER_NUM = -1L ^ (-1L << DATACENTER_BIT);
  private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
  private final static long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);

  // 每部分向左的位移
  private final static long MACHINE_LEFT = SEQUENCE_BIT;
  private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
  private final static long TIMESTMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

  private long datacenterId; // 机房id
  private long machineId; // 机器id
  private long sequence = 0L; // 序列号
  private long lastStmp = -1L; // 上次的时间戳

  public SnowflakeUtil(long datacenterId, long machineId) {
    if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
      throw new IllegalArgumentException(
          "datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
    }
    if (machineId > MAX_MACHINE_NUM || machineId < 0) {
      throw new IllegalArgumentException(
          "machineId can't be greater than MAX_MACHINE_NUM or less than 0");
    }
    this.datacenterId = datacenterId;
    this.machineId = machineId;
  }

  // 产生下一个ID
  public synchronized long getNextId() {
    long currStmp = getNewstmp();
    if (currStmp < lastStmp) {
      throw new RuntimeException("Clock moved backwards.Refusing to generate id");
    }
    if (currStmp == lastStmp) {
      // 若在相同毫秒内 序列号自增
      sequence = (sequence + 1) & MAX_SEQUENCE;
      // 同一毫秒的序列数已达到最大
      if (sequence == 0L) {
        currStmp = getNextMill();
      }
    } else {
      // 若在不同毫秒内 则序列号置为0
      sequence = 0L;
    }
    lastStmp = currStmp;

    return (currStmp - START_STMP) << TIMESTMP_LEFT // 时间戳部分
        | datacenterId << DATACENTER_LEFT // 机房id部分
        | machineId << MACHINE_LEFT // 机器id部分
        | sequence; // 序列号部分
  }

  // 获取新的毫秒数
  private long getNextMill() {
    long mill = getNewstmp();
    while (mill <= lastStmp) {
      mill = getNewstmp();
    }
    return mill;
  }

  // 获取当前的毫秒数
  private long getNewstmp() {
    return System.currentTimeMillis();
  }
}
